Led by Dr. Michael G. Shlipack of University of California, San Francisco, the researchers assessed cystatin C's prognostic value in a group of over 3,000 healthy black and white Americans, aged 70 to 79 years. Serum cystatin C levels were measured and evaluated as a predictor of death during a 6-year follow-up period.

The higher the subjects' initial cystatin C level, the greater their risk of death. Mortality rate increased from 1.7 percent per year for older adults in the lowest one-fifth of cystatin C levels to 5.4 percent per year for those in the highest one-fifth.

With adjustment for other factors — including other diseases and risk factors and markers of inflammation, such as C-reactive protein — risk of death was more than doubled for subjects at the highest versus lowest levels of cystatin C. The relationship between cystatin C and mortality was similar for men versus women and for blacks versus whites.

Breakdowns by specific cause of death found that cystatin C level was a significant predictor of death from cardiovascular disease and of death from other causes. It was unrelated to risk of death from cancer.

In older adults, reduced kidney function is associated with decreased life expectancy. The standard test of kidney function is the serum creatinine level, but the results can be affected by a variety of other factors, including body weight, nutrition, sex, age, and race. In the new study, creatinine level was unrelated to mortality risk, after adjustment for other factors.

Cystatin C, in contrast, is a relatively new kidney function marker that is unaffected by other factors. Previous studies have suggested that cystatin C is a strong indicator of kidney function and mortality in older adults. The new study further evaluated the predictive value of cystatin C in a larger sample of elderly subjects, including both blacks and whites.